The invention concerns a method for extrusion blow-molding a hollow body from a thermoplastic material.
To manufacture hollow thermoplastic bodies, for instance cans or other containers, essentially two kinds of machines are used in practice. One type employs a continuously activated extruder followed by an extruder head from which tubular pre-molded articles exit at the lower end. The other type of machine uses an accumulator head. By means of a thermoplastic material plunger, the thermoplastic or plastic material in the accumulator space is pressed out into a tubular pre-formed article or item. Accordingly, this pressing takes place discontinuously. Because the hollow bodies may assume the most diverse shapes, and where furthermore the pertinent wall zones may be subjected in practice to especially high mechanical stresses, a wall-thickness program is set up over the length of the particular preformed items. Presently the wall-thickness programming is carried out using conventional control means. Equipment is available for programming the wall thickness in the longitudinal direction and furthermore to control the wall thickness over the periphery of the preformed item. Illustratively the transmission cross-section in the extruder head or in the accumulator head is varied over the entire periphery or only in places, whereby a preformed item is obtained, which effects variable wall thicknesses over its length and possibly also in parts of its periphery.
It is understood at the present time that it is important to secure a reliable wall-thickness distribution of the preformed item. This is important when besides the wall-thickness distribution in the longitudinal direction the preformed item also shall have a variable wall-thickness distribution over its periphery. This is the case illustratively when manufacturing bottles or cans. In such cases the programmed points which, following the stretching of a preformed item into a can in the blowing mold, are located in the narrow zones between the upper wall and the lower wall on one hand and the sidewalls on the other hand, are provided with extremely large wall thickness because the wall-thickness program for the radial wall-thickness control allows shifting this excess of material into the areas of more substantial stretching. These extreme points in the preset program can be stretched only when using simultaneously a partial or radial wall-thickness control, because in that case this material also can be shifted in the peripheral direction and no material accumulations may take place in undesired places, for instance near the mold parting lines. The more extremely the program curve must be stretched in particular zones or in relation to particular program points, the more precisely too the material pertinent to those points and subjected to the largest stresses must be present at the proper site in the blow mold. Deviations of more than .+-.1% relative to the height of the blow molded item/body entail significant degradation in quality. Illustratively, warping takes place in the hollow body and substantially lower strain resistance and other degradations in strength are incurred in the finished hollow body.
In addition, about 50% of the manufacturing costs are material costs. The materials paid for by the customer are becoming fewer, and they may not degrade the mechanical properties. Already on grounds of product liability, quality control of the hollow bodies will be mandatory. High quantitative output with reasonable warping is required and assured by keeping constant previously determined wall thicknesses. As a rule poorer mechanical property values cannot be accepted as tradeoffs, and therefore the input weight must be increased and thereby more warping of the hollow body or a lowering in output occurs.
In practice a number of requirements are placed on the hollow body. The table at the end lists on one hand such criteria and on the other the main steps affecting this criteria. The abbreviations used in the table mean the following: PWTC=partial (radial) wall thickness control; SFDR=static, flexible, deforming ring.
To optimize a hollow body in the light of the criteria of this table, in particular to achieve good mechanical properties, and, depending on the difficulties raised by the hollow body, an expert will need from one to five days set-up time at the blow-molding machine. Optimal values then can be achieved only for extreme wall thickness program curve peaks.
The interfering factors can be listed as six different groups and entail the following defects:
(a) Varying extruder output and therefore variable lengths of the preformed items; as a consequence, while the net weight of the hollow body stays constant, the critical points or the critical cross-sectional zones will be displaced within the blow mold.
(b) Varying swelling of the preformed item and thereby variable length of this preformed item; as a consequence, the net weight of the hollow body varies, and the critical points, or the critical cross-sectional zones are displaced.
(c) A varying lower edge at the preformed item in spite of a constant hollow body net weight and a length of the preformed item regulated to be constant at the measurement point results in the absence of a constant gross or preformed weight; as a result, while the hollow body net weight is constant, the critical points, or the cross-sectional zones, are displaced.
(d) In spite of constant weight and length of the preformed item, a varying molding time causes a displacement of the wall-thickness distribution toward the blow mold because of varying shrinkage and/or sagging.
(e) Varying tendency to stretching caused by different plastics, that is different viscosities, temperatures etc. enhances the defects in final stretching for extremely peaked wall-thickness points.
(f) Excessively lengthy compensation of a defect, most of all when starting the machine and when converting to other weights, materials, molds and sizes of hollow bodies etc. entails excessive production shutdown.
The expression "tendency to stretching" means the following behavior of the material. It is constantly found in practice that the material will elongate differently. The main causes are variable distributions in viscosity, different temperatures, different wall thicknesses in the preformed item up to the notch effect and different orientation of the plastic molecule inside the plastic material, even if during the production of the plastic and during further treatment every technical means is employed to arrive at as homogeneous a material as possible.
In order to properly assess the pertinent state of the art discussed below, the possible defects listed above must be taken into account.
The periodical MASCHINENMARKT 1973, vol. 7, pp 118-20 discloses keeping the length of the preformed item as uniform as possible by adjusting the width of the extruder/die slit. Using an accumulator head, the volume of the preformed item is kept constant by means of preset accumulation stroke. If now furthermore the temperature, the output rate and the pressure on the plastic material remain constant, then the gross weight of the produced hollow body shall also be constant. Because of the main interfering factors elucidated below, the lengths of the preformed items will vary and attempts will be made to compensate for these variable lengths by a device seizing the lower edges of the preformed items and subsequently regulating the extruder slit. The goal is always to use the length of the preformed item to move the wall thickness programming preset cavity for the preformed item into the right position relative to the blow mold. However, this cannot be achieved because of defects (c) through (e) earlier mentioned. The operator/expert obliterates the set, optimal wall thickness program, or in plainer words, any peaks in the program curve are rounded off. As a result, differences in wall thickness arise and thereby degraded mechanical properties and warping of the hollow bodies, and most of the time including excessive input weight.
The European Offenlegungsschrift 84 90 0506 discloses a method for regulating the wall thickness of thermoplastic tubular preformed items, the position of these preformed items relative to the blow mold being monitored by a pickup (preferably a photocell) whereupon the preformed items are each widened in a blow mold by means of the inside pressure and lastly the position of each preformed item relative to the blow mold is controlled in relation to at least one predetermined cross-sectional zone of the preformed item. This document further discloses marking in one or more places this preformed item upon expiration of the wall thickness program, where said marking(s) following sensing by the pickup permit the introduction of suitable control steps. However, it is impossible to simultaneously keep the net weight of the hollow body constant or to regulate it.
Apparatus is also known from the German Offenlegungsschrift 29 40 418 to control the weight of a hollow body made by blow extrusion and consisting of a thermoplastic wherein the hollow body weight is determined after it leaves the blow mold and compared to a reference value and wherein, depending on the result of this comparison, the slit width of the discharge is made adjustable to regulate the volume of the material forming the preformed item. Also, a device sensing the length of the preformed item is present which emits a signal to control the motion of the blow-molded parts or of the blow mold. Lastly, control means is included to keep as constant as possible the time within which a preformed item is formed. In this instance an attempt is made for the programmed wall thickness distribution on the preformed item to always assume the proper position relative to the blow mold.
The following documents also are part of the pertinent state of the art, namely U.S. Pat. No. 4,474,716 and the European patent document A 80 10 4933. The apparatus disclosed in these documents is capable within its design limits of compensating fluctuations during production by varying extruder output and by variable swelling of the preformed item as regards long-term. In practice, however, it was found impossible to move the critical points of the wall thickness program and hence the critical cross-sectional zones into the proper position relative to the blow mold. The reasons for this failure are the following: An attempt is made to keep the net weight constant, but this is impossible for a fixed position of the preformed item with the right length if there is a different lower edge in the preformed item (defect (c)). Even when the pickup is located most advantageously at the middle between a concave or a convex lower edge, the required average of the length of the preformed item cannot be achieved, though it is required to keep the preformed item weight constant. In addition to this different edge shape at the lower end of the preformed item, there are also further sources of defects, for instance a less than clean cut, machine vibrations, and oblique tube production in an irregular manner. Because of these defects, the wall thickness points are shifted at or along the hollow body.
Furthermore, the time between sensing the length of the preformed item and the fixation of the preformed item by the blow mold is not constant, that is, there is no constant mold dwell time (defect d)). Also, because of the time-difference T2-TI defect (d) is incurred and thereby, also for a running machine, shifts of the wall thickness points toward the blow mold because of shrinkage and/or sagging.
It must moreover be kept in mind that the preformed item when being widened has a variable tendency to stretch because of differences in viscosity and in material temperature, as a result of which differential stretching of the critical points or critical cross-sectional zones takes place. When using the required partial wall-thickness control, these shifts increase further defect (e), as will be explained in relation to FIG. 7.
Also, the optimization during machine adjustment, which can only be carried out by an expert/specialist, and the levelling procedure, especially in the start-up phase, take much too long in conventional equipment because the superposition of three control loops renders this equipment suitable only for stabilizing production. When starting up, especially on Monday mornings, and until the temperature matches that of the components adjoining the flow duct, or if there are movements of the blow mold, etc., substantial deviations from the reference values will be incurred.
Further deviations will be incurred by changes during the optimization stage and when converting to other materials, weights, sizes of hollow bodies, etc. (defect (f)).
It is also known from the periodical KUNSTSTOFFBERATER 2/1977, pp. 78 through 81 and 87, further 6/1978, pp. 310 through 314 and 319, 320, and also from the Technical Report of the INSTITUT FUER KUNSTSTOFFVERARBEITUNG of the Aachen "Technische Hochschule" by S. Dormeyer, PhD, namely "On Automating Blow-Extrusion" to define the waste-portion length or the waste-portion weight as the measure of the length of the preformed item. The point in this essentially is to achieve a minimum waste-portion length and it is suggested to that end to control both the rate of extrusion and the angular screw speed besides the length of the preformed item. It is suggested in addition to employ a continuous measurement method for the waste-portion length in lieu of the known light sensors located underneath the blow mold. Also, the waste-portion length or the waste-portion weight is considered being a measure of the optimal processing length. In every case the object is to achieve constant waste-portion length and also a constant length of the preformed item. However, such control is by means of light sensors which also serve to measure the rate of extrusion.
The periodical KUNSTSTOFFE 70 (1980), 9, pp. 522 through 524 discloses a device for controlling the weight of blown items or parts. This device operates in such a manner that blown parts made by the blow mold first arrive at a density testing station. The blown parts recognized as being defect-free pass on to a digital weighing scale to sense the blown part weights for weight control purposes. The control loop for the weights in addition to said scale also includes a micro-processor controlled unit and motor potentiometer for automatically adjusting the slit in the wall thickness program as the adjusting means. The scale also includes a photocell that releases the weight value to the computer.
The German Offenlegungsschrift 31 14 371 also is part of the relevant state of the prior art. It clearly instructs the expert of the existing practical difficulties. Return forces in or sagging of the preformed item additionally hamper the association of the wall thickness program with the blow mold. Therefore, in light of the particular operational state and the materials data, a relation is obtained between the measured exit length and an effective length of the preformed item, and this is fed into a comparator. This additional information/input achieves a more accurate association of the wall thickness program on the preformed item relative to the blow mold. But additional drawbacks are incurred, namely, no control of the net weight of the hollow body will ensure. Moreover defects (c) through (e) will arise.
The sources causing defects when controlled by means of the lower edge of the preformed item therefore are essentially the following, both as regards continuous extrusion and accumulator head operation: a messy cut or uneven separation of the preformed item underneath the head, variable support air and support air pressure, machine vibrations, oblique tube runs, back deformation or shrinkage, and because of weight, tube sagging at different times between finishing the preformed item and closing of the blow mold and pre-extrusion in accumulator head operation.